The broad goal of this proposal is to gain a deeper understanding of the adult human mathematical mind by studying its development in infancy. A major goal is to test the shared systems hypothesis that infants rely on analog magnitude representations of number similar to those used by adults, young children, and non-human animals. This will be done in four series of experiments. The first series will use three behavioral methods to test whether infants' numerical discriminations are dependent on the ratio of the numerosities compared rather than the absolute values. The second series will test whether infants attend preferentially to number over other non-numerical stimulus attributes which often covary with number. The third series will test whether infants appreciate the ordinal relations between numerosities and how this ability develops. The final series will explore the role of infants' eye movements in numerical judgments. The research plan is part of the principal investigator's long-term goal of studying both the evolution and the development of numerical cognition. The research proposed broadly relates to health concerns; by advancing an understanding of normal cognitive development which will ultimately provide a context for understanding the cognitive development of abnormal populations. Furthermore this research will contribute to a fuller understanding of the development numerical abilities and may therefore have important implications for teaching topics such as elementary mathematics to children.